I am currently working on the design of a 450V powersupply. I need 2.5A at 450V as the output. The input is 400V from a PFC circuit.

Is an ETD44 core large enough for this application? I calculated that a 30 turn primary at 70kHz gives <0.2 Tesla flux. I have rarely seen people push 1100W through an ETD44 though so I am questioning if there is some other problem I will run into. I can fit enough wire on the core for 500 circular mils per amp so perhaps all will be ok.

I originally wanted to do an LLC topology with the resonant inductors as part of the main transformer. If I go this route I want to use a UCC25600 controller and possibly a gate drive transformer. Switching frequency would be in the range of 80kHz to 120kHz. I found an online LLC calculator and it gave Ls = 25uH, Lp = 126uH, and Cs = 100nF. I also did some calculations by hand and found that the results were similar. I have no idea if it will be practical to reach these values in a real transformer.

One possible complication is that I intend to use a 20uF film cap for the output filter. This cap will have very low ESR on the order of a few milliohms. For this reason the ESR zero that the cap causes will be at a very high frequency. I am unsure if this could cause stability problems.

I also considered using a full bridge but I like the fact that the LLC only needs two transistors.

Does an LLC with ETD44 core sound reasonable for 1100W? With such a high output voltage, will the capacitance of the output rectifiers be a problem?

An ETD-44 at 50khz will give you just enough power for 1.1KW watts. 70khz will give you approximately 1.6KW.

At 70khz you will have 1600G with 26 turns

I played around with transformer margins a little and wire size and 2mm margins on each side will let you have a primary of 5 strands of 25awg, 320 Circ mills each. At 500 Cir Mills / A you would be good for 3.2 pri amps.

Sec turns = 62, and oops, it looks like i made a mistake after this. I mixed up my numbers and gave myself 2x as much room as there really is. Hmm best case i can see is 62 turns of 26awg at 254 Circ Mils for about 1/2 amp at 450vdc.

I would say the secondary may end up driving the transformer core choice.